NEC develops inter-satellite optical terminal

Tokyo—In recent years satellite communication has been the largest and fastest growing sector of space applications and is expected to grow even more rapidly with extended use of the Internet. With this in mind, NEC will soon release a laser-based, optical, inter-satellite communication electro-mechanism, making data transfer even faster and more effective.

Every day, billions of bits of data are transferred by hundreds of low Earth orbit satellites that communicate by a fine-tuned set of laser beams. To communicate, the satellite must be able to capture, process, and then transmit the information from a corresponding satellite while traveling at high speeds. Under these conditions the satellite must switch the optical links simultaneously, so fast acquisition is needed.

Optical terminal showing position on inter-satellite links.

To overcome these technical hurdles, NEC designed an optical communication terminal composed of a course pointing mechanism, to receive data at large off-axis angles, and a fine pointing mechanism, to achieve pinpoint accuracy. According to Nobuaki Takanashi, Manager of the Mechatronics Research Laboratory, "We had to design a mechanism that could achieve a search area over 8°at a distance of 4000 km with a fast acquisition rate of 100 ms. We developed a Wide-Fine Pointing Mechanism that combines the coarse and fine pointing mechanism into one unit, driven by electromagnetic actuators and flexible supports."

The most challenging component was in developing a flexible support design for fine point accuracy that ensured stiffness for low earth orbit reliability, along with the ability to overcome launch vibrations. "We decided the best design for stiffness was to place the mirror on top of a pivot located at its center attaching four thin springs on the sides for rotational flexibility," says Takanashi. "Another important characteristic is that the beam length remains constant because the elasticity between the pivot and thin springs fixes the center of rotation."

The electromagnetic actuators consist of four moving-coil-type motors with long strokes ensuring wide scan range over the entire search area. Typically, piezoelectric actuators are used in communication terminal mechanisms, but the designers discovered that the electromagnetic actuators had a higher scanning range and low voltage requirement, consuming a maximum of 1.23 Watts at the point of highest angle. At 0°the power consumption is zero because no power is needed to adjust the thin springs.

In functioning, the electromagnet actuators force the coils to move through the magnetic field, inducing the thin springs to rotate the mirror attached to its supports. This causes the springs to create a torque rotating the mirror around two orthogonal axes. The pointing mechanism can move at a speed of 140 ms to complete a rotation from 0°to 4°with a vertical movement of a mere 20 microns. Four eddy-current displacement sensors detect the two angles of the mirror ensuring a pointing resolution less than 0.1°rad.

To achieve wide control bandwidth and high stability, a closed-loop system with gap sensor feedback and moving coil driver was designed into the system. The natural frequency response of the Wide-Fine Pointing Mechanism is 50 Hz but the closed loop feedback system allows it to exceed 300 Hz. The mechanism actuates when it receives the input signal from the satellite target and compares it to the real angle detected by the gap sensors, sending an input signal to the coil driver.

At 40 x 40 x 25 mm the entire unit weighs only 250 grams and is suitable for most satellite communication needs, but the company plans on improving the technology. "Currently the optical communication system can transfer information at 50 Mbps," notes Takanashi. "but the company plans to increase the rate to 1 to 10 Gbs while reducing the acquisition time to 60 ms by 2004."

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